The Honda CRF250L is sold by normal Honda dealers, of which you will find more than one in all the larger towns in Thailand, including Hua Hin and Udon.
These shops have been selling small Honda bikes and have taken on the CBR250 and the CRF. they offer sales and service. I do not know which dealer in Udon has the best reputation and you do not see many opinions on these shops posted here, as the standard is reasonably consistant.

Larger capacity Honda bikes will be sold by Honda Big Wing shops, and by coincidence they are opening their Chiang Mai shop today. I will pop along to have a look.